The present invention relates to a magnetometer. In particular, the present invention relates to a low power magnetometer that senses frequency differences to determine magnetic field strength.
A magnetometer measures a magnetic field and produces a signal indicative of magnetic field intensity and/or polarity. Of the magnetometer types, fluxgate magnetometers are popularly used to measure weak static magnetic fields. Fluxgate magnetometers rely upon the saturation of a magnetic core to measure a magnetic field. A great deal of the power consumed by a fluxgate magnetometer arises from the need to saturate the sensor core to detect an external magnetic field.
Frequency-mode magnetometers indicate magnetic field intensity as a function of frequency differences in a sensed signal. Because they do not force the sensor""s core into saturation, this type of magnetometer consumes less power than a fluxgate magnetometer. A frequency-mode magnetometer is disclosed in U.S. Pat. No. 5,239,264 to Hawks, xe2x80x9cZero-Offset Magnetometer Having Coil and Core Sensor Controlling the Period of an Oscillator Circuitxe2x80x9d, (xe2x80x9cHawksxe2x80x9d) which is incorporated herein by reference. FIG. 1 illustrates one embodiment of Hawks"" magnetometer, which uses a relaxation, i.e. LR, oscillator. The magnetometer""s sensor is a wire wound high permeability core 2. The total magnetic field sensed by the magnetic sensor is the sum of the externally applied magnetic field plus the magnetic field induced by the current in the coil. Hawks shows that the frequency of his relaxation oscillator will be a linear function of the externally applied magnetic field if a DC bias current is provided for the magnetometer""s sensor. Without the DC bias current, Hawks"" magnetometer would be unable to differentiate between two magnetic fields of the same magnitude, but of opposite directions. This is because of the symmetry of the permeability curve of FIG. 3. The Schmitt trigger of Hawks"" magnetometer provides the necessary DC bias current. Consequently, on the left side of the permeability curve, increasing magnetic field strength increases permeability and thus, the sensed frequency. Whereas on the right side of the permeability curve, increasing magnetic field strength reduces permeability.
To obtain a zero-compensated reading from his magnetometer, Hawks operates on both sides of the permeability curve in a mirror-image fashion. This requires setting the level of the DC power supply, Vs, equal to the sum of the Schmitt trigger""s high and low trip points, VH and VL. In other words, Vs=VH+VL.
A switch, illustrated in FIG. 1, controls the polarity of bias current within Hawks"" magnetometer. While the operation of the oscillator is nearly identical regardless of the input level, throwing the polarity switch changes the net current through the bias resistor 4. The differences between the current for negative and positive bias polarities are illustrated in FIG. 2. While the waveforms are virtually identical in the absence of an externally applied field, the current between the two polarities is shifted by an amount Is. This shift can place the biasing in the opposite half of the curve by changing the level of Bias Polarity input.
Hawks obtains a zero-compensated output by taking two readings with his magnetometer. One reading is taken on one side of the permeability curve and another reading is taken on the other side of the permeability curve, one reading is subtracted from the other to obtain the final zero-compensated output.
Performance optimization of Hawks"" magnetometer is difficult because of the inter-relationship between the DC bias current, maximum drive current, and output frequency arising from the use of an LR oscillator. For example, the maximum drive current can be changed by changing the value of R, however, this also changes the output frequency. (See Equation 6 in column 5 Hawks.) Such a change in output frequency may be unacceptable because, as illustrated in FIG. 4, the permeability of the sensor core is frequency dependent, diminishing as frequency increases. Hawks cannot resolve this problem by changing the inductance of the sensor because that will also modify the DC bias current and maximum drive current. (Again see Equation 6 in column 5 of Hawks).
An additional drawback of the Hawks"" frequency-mode magnetometer is its power consumption, which while less than that of a fluxgate magnetometer, is nonetheless less than desirable for battery powered applications because the power required to charge the magnetic sensor is not conserved within the magnetometer.
Thus, a need exists for a low power frequency-mode magnetometer whose operation can be easily optimized to operate in a region of maximum permeability change on the permeability Vs magnetic field curve of the magnetometer""s sensor.
The magnetometer of the present invention offers a number of advantages over those of the prior art. As compared to prior frequency-mode magnetometers, the present magnetometer is easier to operationally optimize, consumes less power, and is inexpensively manufactured.
Briefly described, the present invention is a low power, frequency-mode magnetometer, which includes an LC oscillator, a bias resistor, and a polarity switch. The LC oscillator produces an output signal indicative of both a magnitude and a polarity of a magnetic field. The LC oscillator includes an inverting amplifier and a Pi network including an inductor and two capacitors. The inductor is realized as a coil wound about a high permeability core. The inductor is coupled between an input node and an output node, as is the inverting amplifier. The inverting amplifier has a first trigger voltage and a second trigger voltage, which are symmetrically located with respect to one half of a DC bias voltage. The bias resistor is coupled between the input node of the LC oscillator and a first node, and controls the DC bias current to the inductor. The switch switches the first node between ground and the DC bias voltage.
Other objects, features, and advantages of the present invention will be apparent from the accompanying drawings and detailed description that follows.